In order to improve the fuel efficiency of an automobile, the power steering pump which continuously circulates a hydraulic fluid, is replaced by an electrical motor that is actuated in response to the manipulation of the steering system.
Accordingly, the vehicles engine is no longer providing a driving force to the power steering pump. This results in a lower overall load upon the engine which will improve fuel efficiency.
An electric motor is used to provide a steering assisting force in response to manipulation of the steering system. One such motor is a brushless DC motor which is mechanically coupled to the steering column in order to provide the assisting force.
A brushless DC motor requires electronic commutation of its armature currents. This is typically accomplished, for a trapezoidal back EMF, three-phase motor, by means of a bridge switching circuit containing six semiconductor switches. (FIGS. 3 and 4).
When a predetermined upper switch and non-series lower switch are simultaneously conducting, an armature current flows through two of the three-phase windings, in series, to electromagnetically interact with the permanent magnet motor and develop torque in a predetermined direction.
Commutation to a different pair of armature windings is accomplished by turning off one of the switches and turning on a different non-series switch of the same level (upper or lower). A control provides switching signals to the switch gates in the proper sequence and with the proper timing.
Accordingly, motor operation is controlled by activating the switches in a predetermined pattern.
The electric currents in the activating phases are controlled, usually by a pulse width modulation at a higher frequency, to control motor torque. This modulation, in combination with the inductance of the windings, produces an average motor current, and thus a smooth motor torque. However, this is not the case during commutation.
During commutation, where the phases are turned off and on abruptly, the rise and fall of the phase currents are controlled only by the network properties of the motor and switching circuitry. The forcing function for the phase turning off is not the same as it is for the phase turning on, and one of the circuits (rising or falling) will change faster than the other.
The unequal current in the phase turning off and phase turning on can produce a disturbance in the average motor current, which will cause a variation in torque, at each commutation event. In addition, such disturbances can also drive the motor structure to generate audible noise.
Moreover, and in some applications where the motor structure is positioned within the passenger compartment of a vehicle, the audio level of these noises may reach an objectionable level. In particular, use of such a control and motor in an electric power steering system in modes of operation which produce high phase currents at low motor speed can produce an annoying clicking noise at commutation. This audible noise has been referred to xe2x80x9czipper noisexe2x80x9d.
Accordingly, it is desirable to control the phase currents of such a motor in order to reduce or eliminate such audible noises.
The motor control of this invention modifies motor commutation events to reduce noise by providing overlapping activation of the switch being deactivated with the switch been newly activated so that three switches are temporary activated. The newly activated switch is pulse width modulated with a motor torque controlling duty cycle, and the switch being deactivated is pulse width modulated synchronously with the newly activated switch but with a duty cycle decreasing from the torque controlling duty cycle. The decreasing modulation of the switch being deactivated allows a slower current decrease in the phase turning off to reduce or eliminate the disturbances, and thus the noise. The decreasing modulation is preferably exponential in manner, with a time constant varying inversely with motor speed so that the duty cycle reaches a predetermined minimum in a predetermined maximum number a motor electric degrees.
The decreasing modulation may be switched on at low motor speeds where its noise reduction is required and switched off at higher motor speeds where the potential for interference with commutation is greater. The decreasing modulation is applicable in a dual switching mode in which each activated switch, except the switch being deactivated in commutation, is pulse width modulated at the torque controlling duty cycle. The decreasing modulation is alternatively applicable in a modified single switching mode in which the activated switch of one of the upper or lower groups is always closed continuously during its activation, except for commutation, during which the switch remaining on, which alternates between the groups, is closed continuously. The control is preferably responsive to motor current and/or motor speed to apply each switching mode in the motor speed/torque region to which is best suited. Preferably, control stability is improved by a sample/hold circuit and a motor speed controlled variable forcing function generator to augment a sense current feedback signal during the portion of a commutation event in which the early opening of one of the switches causes current to recirculate in the bridge and not be sent by the current sensor.
In yet another embodiment, the time constant is further modified in order to reduce or eliminate disturbances. The further modification of the time constant is dependent upon the input current command.